Prosthesis comprising an expansible or contractile tubular body

ABSTRACT

A prosthesis for transluminal implantation comprising a flexible tubular body which has a diameter that is variable by axial movement of the ends of the body relative to each other and which is composed of several individual rigid but flexible thread elements each of which extends in helix configuration with the center line of the body as a common axis, a number of elements having the same direction of winding but being axially displaced relative to each other crossing a number of elements also axially displaced relative to each other but having the opposite direction of winding; and method for transluminal implantation.

This application is a continuation of application Ser. No. 219,800 nowabandoned, (cont. of No. 946,064 now abandoned; cont. of No. 571,549)now U.S. Pat. No. 4,655,771, filed 7/15/88 (cont. date of 12/24/86;cont. date of 12/7/83).

TECHNICAL FIELD

The present invention relates to a prosthesis which can be appliedwithin or replace part of for example blood vessels of the body of aliving animal or a living human or in some other difficultly accessibleplace. The prosthesis includes a flexible tubular body, the diameter ofwhich can be decreased or increased. The invention is partially usefulfor mechanical transluminal implantation by means of an expandedself-fixating prosthesis for blood vessels, respiratory tracts or thelike. By means of the device of the present invention also the innerwalls of damaged blood vessels or other organs may be lined withartificial tissue.

In surgical and other medicinal techniques there is sometimes a need ofinserting and expanding a device in for example blood vessels, urinarytracts or other difficultly accessible places which has for its functionto support the vessel or tract which can be left in a position.

The device according to the present invention can be used in manymedicinal applications and, as examples, there may be mentionedutilization in different types of aneurism reflected by some form ofvessel widening, or the opposite, stenosis, which involves contractionof blood vessels. Thus, more specifically, the invention can be used tosupport and keep open vessels of venous systems, to close pathologicalvessel failures, to bridge pathological vessel dilatations and rupturesin interior vessel walls or to stabilize bronchial tubes and bronchi.The device according to the present invention may also be designed toact as a filter for thrombosis, for example by application in Vena CavaInferior to prevent the formation of lung emboliae. The invention isparticularly suited to be used as a prosthesis, for example a graft, forapplication in blood vessels or other tubular organs within the body. Itshould, however, be observed that the invention is not limited to theapplications mentioned which must be considered as solely exemplifying.

BACKGROUND ART

In U.S. Pat. No. 3,868,956 there is described a device which afterinsertion into for example blood vessel may be expanded. The active partof this device is based on the use of metal alloys having so-called"memory function", i.e. a material which when heated will recover itsinitial configuration. In this prior art the heating of the material isprovided by electrical heating, the device being inserted at thelocation of interest. However, this known technique possesses theessential disadvantage that electrical resistance heating must takeplace in connection to surrounding sensitive tissue which may be damagedin the heating. It is true that it is stated in the patent specification(cf. col. 3, lines 42-48) that when inserting the device into a bloodvessel the patient's blood functions as a cooling medium. However, alsoblood is a heat-sensitive material which when heated can be subjected toundesirable coagulation.

SUMMARY OF THE INVENTION

The present invention has for its purpose to provide a radiallyexpansible and contractile prosthesis whereby the drawbacks of the knowntechniques are avoided.

The present invention is based on utilization of a prosthesis comprisinga flexible tubular body the diameter of which can be changed by axialmovement of the ends of the body relative to each other. In a preferredembodiment the body takes a radially expanded position by itself. Whenit is left in an unloaded condition free of external forces in radialdirection. The body is composed of several individual rigid but flexiblethread elements each of which extends in helix configuration with thecenter line of the body as a common axis. A number of elements have thesame direction of winding but are displaced axially relative to eachother. The said number of elements having the same direction of windingmeet under crossing a number of thread elements also axially displacedto each other but having the opposite direction of winding.

To obtain the desired function the axially directed angle betweencrossing elements is suitably greater than about 60° and is preferablyobtuse, i.e. more than about 90°. This state of the body refers to itsstate in radially unloaded condition.

It is preferred to arrange the crossing thread elements in such a manneras to form a sort of braided configuration which may be varied asdesired and for example imitate some known type of weaving, for exampleaccording to the principle of a plain weave. The object of this is toimpart to the tubular body the necessary stability. If the number ofelements in the flexible tubular body is designated n it is preferredthat n varies from about 10 and up, for example to about 50. Theelements of the tubular body are preferably arranged symmetrically, i.e.the number of elements in each direction of a winding is n/2. It shouldbe observed that in this connection when referring to the number ofelements in the tubular body reference is always had to elementsintended to maintain the supporting function of the body. The number ofelements n is selected in accordance with the diameter of the body, thediameter of the element, the material of the element or other factors.Quite generally, the greater the diameter of the body with a givenelement material, the more elements should be used to give the necessarystability of the body.

The flexible tubular body according to the present invention has beenfound to be quite suited for use as a prosthesis for transluminalimplantation in blood vessels or other similar organs of the livingbody. The tubular body is inserted into place in the organism incontracted state, i.e. with reduced diameter. After the tubular bodyaccording to the invention has been inserted into position it issubjected to expansion and can stay in place in expanded state byself-fixation if the diameter of the body in unloaded condition isselected somewhat larger than the diameter of the surrounding wall. Thisresults in a certain permanent pressure of engagement against the innerwall so as to ensure good fixation.

This implantation process is quite simpler and less risky than the knownimplantation technique involving a non-expansible prosthesis. Theradially contracted prosthesis which e.g. is inserted through the wallof the vessel at a distance from the implantation site will be fixedwithout the need for conventional removal of the parts of the organ tobe replaced. In this manner the blood flow can be maintained even duringthe implantation which calls for a short period of time. The prosthesisneeds not be stitched to the vessel and already after a few days it hasbeen definitely fixed to the body by means of natural tissue growth andafter a few months the tissue growth is complete and the inside wall ofthe prosthesis is covered by new natural tissue.

The flexible tubular body can be brought to expand radially in severalways. It has been found for many reasons that it is preferred that thebody has the property entering into radially expanded and unloadedposition by itself. The expanded state of the body may be dependent onthe inherent rigidity of the threaded elements, but it may also becontrolled by elastic strings, band or membranes which are arranged inconnection to the mantle surface of the body and extend axially alongsame. By their elasticity these strings, bands or membranes result inaxial traction of the body, i.e. to bring same to take an expandedstate.

An alternative way of imparting properties to the body through which ittends to take a radially expanded position is to attach the elements toeach other at the points of crossing thereof in a suitable manner, forexample by some form of welding, gluing or the like.

The elements forming the flexible tubular body should be made of amedicinally acceptable material, for example plastic or metal, and theyshould possess certain springiness or rigidity combined with suitableelasticity. The elements may be built up as monofilaments, for examplepolypropylene, dacron or other suitable plastic or constituted by acomposite material. They may also be made from some suitable medicinallyacceptable metal, for example steel.

The free ends of the thread elements of the tubular body can be modifiedor protected in several ways. The alternative in which no free ends atall are present is the alternative to make the tubular body as a wholeof one coherent element. The alternative which is most closely relatedto that is the case where the free ends of a body resulting fromsevering a long string are connected with U-shaped members which areattached to the ends of the elements pair-wise in a suitable manner, forexample heat welding, gluing or the like. In this manner elements of thesame direction of winding or elements of the opposite direction ofwinding can be attached to each other two and two.

An alternative to these embodiments is to weld together the points ofcrossing in a ring around the material by electric resistance heating orthe like before severing the string, severing then taking place adjacentto and just outside the welding site. The ends then extending outsidethe welding area may be folded inwardly towards the interior of the bodywith light plastic deformation, for example through controlled heating.Yet another alternative consists in bending the free ends of theelements to form loops.

As previously indicated the tubular body according to the presentinvention is suited for use as so-called graft. In this case the bodymay function as a graft namely if it is made of elements of suchcharacter as to impart by themselves the desired density and porosity tothe body to function as a graft whereby at least a number of theelements may be made of polyfilament materials or the like. Thealternative of the elements themselves imparting the desired density tothe body is to apply some sort of surface layer to the body, for exampleof plastic or other suitable material. By applying such surface layerthe crossing points may at the same time be fixed as indicated above soas to make the body tend to take an expanded position.

Outside or inside or amalgamated with the body there may also bearranged a separate sleeve or membrane. This can be constituted by astocking of porous web surrounding the body which can be implantedtogether with the body. In this case the stocking may either bystretchability in the web or by overlapping folding or in anothermanner, for example by being built up in accordance with the sameprinciple as the body from a plurality of thread elements, be adjustableto the body in connection with the expansion thereof. It is alsopossible to conceive the use of some form of tricot type product orcrimped fibre textile. When using such a separate member it is preferredthat it is axially fixed relative to the body so as to end up in theright position when applied in a large vessel or the like.

The expansion or contraction of the tubular body can be provided by adevice with means which are arranged to elongate or shorten the body.Such means may be designed in many ways, for example so that theirconstruction allows axial movement of the ends of the body relative toeach other to reduce or increase the diameter of the body. The deviceshould include gripping members capable of gripping the ends of the bodyand axially moving same relative to each other. The gripping membersshould be arranged so as to be releasable after the application of thebody at the desired site so that the device except for the body can beremoved from said site. Alternatively, the device may include a flexibletube within which the tubula body is intended to be placed in contractedstate, and operating members by means of which the body under expansionthereof can be pushed out of the tube to be applied at the desired site.

Other characterizing features are obvious from the appended patentclaims.

EXAMPLES

The invention will in the following be described by non-limiting butexemplifying embodiments in connection to the appended drawing. In thedrawing these embodiments are illustrated and:

FIG. 1A and FIG. 1B show diagrammatically a side view and an end view,respectively, of the flexible tubular body according to the invention;

FIG. 2A and FIG. 2B show the same tubular body as in FIG. 1 but incontracted state;

FIG. 3 and FIG. 4 show one separated thread member of the body, the bodybeing shown in contracted and expanded state, respectively;

FIG. 5 shows diagrammatically an assembly incorporating the tubular bodyaccording to the present invention;

FIG. 6 shows in an enlarged view part of the assembly of FIG. 5;

FIG. 7 shows an alternative embodiment of the tubular body;

FIG. 8 shows the tubular body designed as a combined graft and filter;

FIG. 9 shows the tubular body used as a graft in connection to aneurism;

FIG. 10 shows a diagram of the diameter (D) of the body as a function ofthe angle α and of the elongation of the prosthesis in %;

FIG. 11 shows diagrammatically an alternative assembly for manipulatingthe prosthesis of the invention.

In FIGS. 1a and 1B there is shown in an example of a prosthesis in theform of a cylindrical tubular body generally designated 1. As is clearfrom FIG. 1A the mantle surface of body 1 is formed by a number ofindividual thread elements 2, 3 etc. and 2a, 3a etc. Of these elementselements 2, 3 etc. extend in helix configuration axially displaced inrelation to each other having the center line 7 of body 1 as a commonaxis. The other elements 2a, 3a extend in helix configuration in theopposite direction, the elements extending in the two directionscrossing each other in the manner indicated in FIG. 1A.

The diameter of a tubular body built up in this manner can be varied ifthe ends of the body are axially displaced relative to each other in thedirection of the center line 7. In FIG. 2A there is illustrated how thetubular body 1 according to FIG. 1A has been given reduced diameter bymoving the ends 8, 9 away from each other in the direction of thearrows. FIG. 1B shows the diameter of the tubular body an expandedstate, whereas FIG. 2B shows the diameter of body 1 in contracted stateafter the ends 8, 9 thereof have been moved away from each other.

FIGS. 3 and 4 show a detail picked from FIGS. 1 and 2, morepracticularly one single thread element of the tubular body 1 and howits helix configuration will be changed in connection with the change ofthe length of the tubular body 1.

In FIG. 3 the individual element 10 corresponding to element 10 of FIG.2A is shown. The diameter of the helix is d₁ and the length of theelement is l₁. In FIG. 4 the same element 10 is shown after the tubularbody has been expanded to the state shown in FIG. 1A. The diameter ofthe helix has now increased and is designated d₂, whereas the length hasdecreased and is designated l₂.

The tubular body 1 can be expanded in a number of ways. As previouslymentioned it is preferred that the body inherently has the property oftaking expanded position by itself in unloaded condition. In the presentdisclosure the expression "expanded position" always refers to radialexpansion, i.e. a state with a large diameter of body 1. Theself-expanding property can be obtained by providing the body withstrings or bands extending parallel and axially with the mantle surfaceof the body. An example of such embodiment is shown in FIG. 7 where thetubular body 1 is provided with axial strings or bands 11. These stringsor bands 11 are suitably made of an elastic material and they are fixedto the elements of the tubular body 1 in a suitable manner and with thebody in expanded state. Now, if the tubular body 1 is axially elongatedby removing the two ends thereof from each other the elastic strings orbands 11 will be stretched. After removal of the tensile force from thebody 1 the elastic strings or bands 11 will compress the body 1 in anaxial direction resulting in a corresponding increase of the diameter ofthe body.

The tubular body 1 can be provided with the same tendency to takeexpanded position by fixing the elements 2, 3 etc; 2a, 3a etc. at thecrossing points 5, 6 (FIG. 1), as previously mentioned. Another way ofproviding this effect is to provide for an interior or exterior tubularelastic member, for example of a thin elastomer, which is attached to atleast both ends of the tubular body.

In FIG. 5 there is shown a device generally designated 18 to enableinsertion of the tubular body 20 in contracted and elongated state atthe desired site of for example a blood vessel. The tubular body 20surrounds the forward tubular part 19 of apparatus 18 and is attached atboth ends thereof to gripping means 21 and 22. The forward tubular part19 of the apparatuus is connected to an operational member 24 through aflexible tubular means 23. By means of operational elements 25, 26 and27 of the operational member 24 the gripping means 21 and 22 can becontrolled in a desired manner.

In FIG. 5 there is shown diagrammatically how apparatus 18 with thecontracted tubular body 20 has been inserted into for example a bloodvessel which in the figure is shown with dashed lines and designated 28.Operational member 24 is connected with gripping member 22 in such amanner that when the operational means 26 is moved forwardly to position28 shown with dot and dash lines a gripping member 22 is displaced in acorresponding manner to the dot and dash line position 30. As a resultthe end of tubular body 20 has been moved from position 22 to position30, whereas in this case the other end of the body remains in position21. At the same time the diameter of body 20 has increased and when theend has reached position 30 the body 20 is expanded, i.e. it has beenbrought into contact with the interior wall of the vessel and has takendash-dotted line position 31. Since both ends of the tubular body 20still are held by members 21, 22 body 29 in expanded state takes aballoon-like shape.

Operational means 27 is also connected with the gripping member 22 bymeans of a part, for example a wire, running in tubular member 23. Inthis manner gripping member 22 in its position 30 can be manoeuvred byaxial displacement of operational member 27 to release the end of thebody 20. In the same manner manoeuvring means 25 which is connected togripping member 21 can release the forward end of the tubular body fromgripping member 21 by axial displacement thereof. The ends of theelastic body 20 are thereby immediately subjected to movements relativeto each other to provide for expansion and the prosthesis takes itsexpanded cylindrical shape in the interior of the blood vessel.

In FIG. 6 there is shown more in detail and in enlargement theconstruction of the forward tubular part 19 of device 18. The tubularbody 20 with its both ends 32 and 33 surround a thin-walled flexibletube 34 running inside and concentrically to an outer flexible tube 35,the two tubes of which form the tubular member 23 in FIG. 5. At thefront part of the inner tube 34 an annular member 36 is arranged, intowhich the end 32 of tube 20 is inserted. In a corresponding manner theend 33 of tube 20 is inserted into an annular member 37 which is axiallydisplaceable in relation to the tube 34 surrounded by ring 37. At thefront part of tube 34 there is provided an interior gripping member orlatch 38. Latch 38 which is suitably made of spring steel, has a forwardpointed part 39 bent under about right angle. This part 39 extendsradially outwardly through a hole in tube wall 34. It can move in radialdirection under the influence of a ring 40 which is axially movable andarranged inside tube 34. Ring 30 is connected to a wire 41 through whichby axial displacement latch 38 can be moved in a radial direction. InFIG. 6 latch 38 is shown in such position that its pointed part 39 hasperforated the end 32 of body 20 and thus maintains said end inposition.

In the corresponding manner another latch 42 is arranged to hold fromthe outside the end 33 of the tubular body 20 by its pointed part 43.This latch 42 which is attached to the outside of tube 35 can be movedin radial direction by means of a ring 44 arranged about tube 35 andattached to a cable 45 extending between tubes 34 and 35. Cables 44 and45 are connected to the operational means 25 and 27, respectively, inFIG. 5.

When the attached and axially extended tubular body 20 shall be releasedfrom the remaining part of the device after the radial expansion of thebody this takes place by releasing the pointed parts 39, 43 of latches38 and 42, respectively, from the ends of tubular body 20 by actuatingrings 40 and 44 through operational members 25 and 27 via cables 41 and45 so as to deflect latches 38 and 42. Ends 32 and 33 of body 20 willthen be released by axial displacement of the tubular part 19 of theapparatus. As is clear from FIG. 6 the front end of the apparatus isprotected by a hub or casing 46 attached to ring 36.

As previously indicated the expansible tubular body finds severalapplications within surgery. For example, in the embodiment shown inFIG. 1 it can be utilized for supporting vascular walls. In FIG. 8 thereis shown a modified embodiment of the flexible tubular body. In thisembodiment the body consists of a cylindrical circular part 53 which atone end thereof changes to a diminishing part or end 54 also built upfrom thread elements. This device has been found to be suitable for useas a sieve or filter to prevent thrombosis. The device shown in FIG. 8can be applied at the desired location within a blood vessel, forexample Vena Cava Inferior, for the purpose of preventing lung emboly.Previously known filter means intended for application within bloodvessels for the purpose of catching thrombosis are associated with thedisadvantage that they are permanently attached in the blood vessel bypointed ends or latches or the like, positional correction or removal ofthe filter not being possible. An example of such device is described inU.S. Pat. No. 3,540,413. The device according to the present inventioncan be inserted into Vean Cava with great precision and it does notinvolve any risk for damages on surrounding vascular wall which is thecase with known devices used today in surgery for the same purposes.

In FIG. 9 there is shown a tubular body according to the presentinvention for use as a graft. In this case body 55 has a much denserwall than the embodiment shown in FIGS. 1 and 2. This denser wall can beobtained by weaving an elastic yarn between the supporting threadelements 2, 3 etc.; 2a, 3a etc. of FIG. 1. In this manner a wall havinga controlled porosity can be obtained. This tubular body having a moreor less porous wall is thus a sort of expansible graft and has versatileuse.

In the application shown in FIG. 9 body 55 is implanted into an aorta 56wherein there is an aneurism 57 in the form of a widening of thevascular wall. In view of the fact that the expansible body or graft 55can be inserted at a distance from the damaged location of aorta andthen located in the middle of the aneurism the latter will be bridgedand need not be operatively removed. In FIG. 9 it is also indicated thataorta is a conical blood vessel. Therefore, the procedure in this casewill be that the prosthesis in the form of a graft is inserted with aninstrument, for example in accordance with FIG. 5. After being locatedthe graft or body 55 is expanded. In view of the conical configurationof aorta the surgical techniques will be as follows.

The front end 31 of graft 55 according to FIG. 5 is inserted somewhatfurther into aorta than the location it shall take after terminatedoperation. This position 59 is indicated in FIG. 9 with dash-dottedline. The other end 22 of the axially extended graft 55 according toFIG. 5 is carried up to the final position corresponding to position 60of FIG. 9 before the radial expansion. Since this part of aorta has asomewhat smaller diameter than the diameter in front of the aneurism asseen upstream in relation thereto the prosthesis cannot expand more thanthe dimension corresponding to the diameter at end 60. This is, however,alleviated by then moving the other end of graft 55 by means of thefront part of the instrument from position 59 to position 58 so thatthis end of the graft can expand sufficiently to engage this part of thevascular wall.

In FIG. 11 there is shown another embodiment of the assembly for use inexpanding the tubular body.

This assembly constitutes a flexible instrument intended to introducethe tubular body in contracted state into for example a blood vessel andthen to expand the body when located therein. The parts of theinstrument consist of an outer flexible tube 61 and a concentric alsoflexible inner tube 62. At one end of the outer tube an operationalmember 63 is arranged. Another operational member 64 is attached to thefree end of inner tube 62. In this manner the inner tube 62 is axiallydisplaceable in relation to the outer tube 61. At the other end of innertube 62 a piston 65 is attached which when moving runs along the innerwall of outer tube 61.

When the instrument is to be used the tubular expansible body 69 incontracted state is first placed inside tube 61, the inner tube 62 withthe piston 65 being located in the rear part 66 of outer tube 61. Thestarting position of piston 65 is shown by dashed lines at 67 in FIG.11. In this manner part of tube 61 is filled with the contracted tubularbody 69 in the starting position.

During implantation the flexible tubular part of the device is insertedto the location of a blood vessel intended for implantation. Member 64is then moved in the direction of arrow 68, the contracted body 69 beingpushed out through end 70 of tube 61, the part of the tubular body 69leaving tube end 70 expanding until in its expanded position 71 it isbrought to engagement with the interior of vascular wall 72. The tubularbody 69, 71 is for sake of simiplicity shown in FIG. 11 as twosinus-shaped lines. To the extent that the expanded body 21 comes intoengagement with vascular wall 72 tube end 70 is moved by moving member63 in the direction of arrow 73. The contracted body 69 is moved by thepiston 65 pushing against one end of the body. Thus, the implantationtakes place by simultaneous oppositely directed movements of members 64and 63, the displacement of member 64 being larger than that of member63. When the contracted body 69 has been fully removed from the tube 61the expansion is terminated and the instrument can be removed from thelocation of the operation.

The embodiment according to FIG. 11 has the great advantage that theconstructional details are quite simple and can be operated with highreliability. The instrument shown is also suitable for implantation ofhelices with very small diameters. As an example there may be mentionedthat experiments have been performed with a tubular expansible bodyconsisting of crossing thread elements, the contracted diameter of thebody being only 2 mms and the expanded diameter 6 mms. It is also fullyconceivable to implant expanded bodies with even smaller diameter. Theinstrument according to FIG. 11 may also advantageously be used forimplantation of bodies in the form of grafts of a very large diameter.

In implantation of long bodies it is conceivable that the resistance indisplacing same in tube 61 becomes too high. In this case it may besuitable to replace piston 65 at the front end of tubes 62 with movablejaws or latches which operate in such a manner that when tube 62 isbrought forward in the direction of arrow 68 the latches engage theinner side of body 69, the body being brought forward. When tube 62 isbrought back in the direction of arrow 73 the latches are released. Inthis manner body 69 can be moved forwardly by a pump-like motion of tube62.

Many embodiments of the different members shown in FIG. 11 are, ofcourse, conceivable. Thus, it is possible for example to simplifyimplantation for the surgeon by controlling the relative motion betweenmembers 63 and 64 in a mechanical manner.

It is essential that the expansible body possesses certain elasticproperties in order to enable successful implantation. For example, whenthe body is inserted to keep blood vessels open or is implanted as bloodvessel prosthesis it should have elastic properties which are as similaras possible to those of the blood vessel of the living body. The bodymust also remain fixed against the surrounding organ, for example theblood vessel, during the stress and strain the organ is subjected to.The body must at the same time be elastically resilient radially andaxially so as to have for example sufficient adaptability to followpulsation of the blood or the bending of a limb. The body shall alsohave sufficient inherent rigidity so as to maintain its shape at forexample external pressure and must have sufficient strength to resistinternal pressures.

In order to obtain these properties it is suitable carefully to selectand adapt materials and dimensions on the thread elements of the body tothe actual area of application. In addition to the obvious requirementthat the material of the thread elements shall be compatible with thetissue, i.e. inter alia result in minimum reaction of rejection, benon-toxic and enable cell growth, it may be generally said that thematerial should be rigid and elastic and not plastically deformable toany significant extent. The material may for example be monofilaments ofpolyesters, polyurethanes, polycarbonates, polysulphides, polypropylene,polyethylene, polysulphonates, stainlesss steel, silver. The diameter ofthe monofilament should suitably lie within the range 0.01 to 0.05 mms.

It has been found that in certain cases it is important that the angle αbetween the thread elements of the body, for example between 2 and 2a ofFIG. 1A, when the body is expanded or is in an unloaded or nearlyunloaded state is sufficiently large, inter alia to meet the aboverequirements. It has been found that the greater the angle α the higherthe stability of the body under external pressure. The ideal from thispoint of view would be 180°, which is not practically possible. Theangle as shown in FIG. 1A is about 160°, which normally is close to theupper limit.

In order to change the diameter of the body it is required, asindicated, that both ends of the body are axially displaced relativeeach other. In FIG. 10 there is shown the general relation between thismovement. The change in per cent in diametter when the ends are movedaway from each other has been plotted along the y-axis and along thex-axis the correspondong change in per cent in length expressed aselongation. Along the x-axis there has also been plotted the angle α asa function of the diameter of the body.

As is seen from FIG. 10 the relative diameter reduction is small at theoutset of the elongation process and the diameter has been reduced tothe order of 90% when the elongation is 100% referring to the startingposition where the angle α is as close to 180° as is practicallypossible. At an elongation of 200% the diameter reduction is 75%corresponding to an angle α of 100°. The diameter reduction will then beaccelerated at increasing elongation. Thus, an elongation increase from250 to 300% results in a diameter reduction from 60% to 30%, i.e. arelatively large diameter change at a relatively small elongation.Within this range the angle is reduced from about 70° to 40°. Asindicated above it is in some cases desirable that the expanded bodytakes a position which is as far to the left on the curve of FIG. 10 aspossible, i.e. the angle α should be as large as possible. Since theimplanted body must engage against the vascular wall with certainpressure in order to remain fixed the diameter of implantation must besmaller than the diameter at free expansion.

When using expansible bodies according to the invention for implantationin blood vessels or other tubular organs the necessary expansion forcesmay be provided for example by elastic means, such as longitudinallyextending elastic strings fixed at the crossing thread elements of helixconfiguration. By selecting a large angle α when the elastic means arefixed to the elements the requirements previously mentioned may be metin a simple manner.

The reason why a large value of the angle α is often desirable is thefact that the elastic properties of the prosthesis are impaired withdecreasing angle. Under for example exterior pressure in a radialdirection the resistance to deformation is small and there is a risk forlocal axial displacement between prosthesis and vascular wall, which canprevent cell growth at the site of displacement. Another reason forselecting a high value of the angle α is in those cases where a highexpansion ratio is desired, i.e. a high ratio between diameter of theexpanded body and the diameter thereof in contracted state. In order toobtain for example expansion ratio over 2 up to about 3 the angle αshould exceed about 120°. The selection of the angle α is also dependingon the material of the thread elements of the prosthesis. If a plasticmaterial has been selected too small an angle α results in too highresiliency in radial direction. In some other cases it may, however, bedesirable to select a smaller angle α, namely in those cases wherepronounced radial yield is desired.

Another case where a high value of the angle α might be desirable isapplications wherein the prosthesis as applied will be subjected to abending. The resistance to flattening of the prosthesis will thus behigher the larger the angle α. Thus, it is suitable to select an angle αwhich is more than about 60°, and an obtuse angle α could beparticularly suitable. To provide for high resistance to externalpressure or to enable high expansion ratios it is preferred to select anangle α of at least about 120°.

From FIG. 10 it is clear that the body must be highly extended whenusing large angles α. To enable transluminal implantation throughpasseges of small diameters the elongation starting from large angles αmay be substantial and can be up to 300% and even more.

When implanting for example vessel prostheses or similar devices, forexample to keep blood vessels open, it is as a rule desirable to reach apressure against the surrounding vascular wall which is at least about100 mm Hg. There is also a highest pressure which must not be exceeded.This highest pressure varies from case to case but should not exceedabout 500 to 1000 mm Hg when used as a vascular prosthesis. If thedesired pressure will be provided by longitudinally extending elasticmembers or an elastic sleeve or membrane the necessary pressure forfixation can be obtained with reasonable forces when selecting a largeangle α which is advantageous. Thus, calculation show that in smoothcylindric engagement between vascular prosthesis and surroundingvascular wall there is required a total force of a few Newtons(˜0.1-0.2kp) to obtain fixation if the angle α is 150°-170°. This factalso contributes to reduced risk of displacement of the implantedprosthesis under external pressure since the frictional forces arisingare sufficient to prevent such displacement. If the angle α is forexample 45° there is, however, required a force of about 10-20 Newtons(1-2 kp) which is practically disadvantageous.

In order that the prosthesis of the invention shall operate in asatisfactory manner, inter alia to give the necessary fixation whenapplied, such requirements must be met in regard to the elastic materialresulting in the necessary expansive force. The material must alsoresult in acceptable adherence to the thread elements of the body andmust, of course, be biologically acceptable for implantation. Thematerial shall thus have a low module of elasticity and should present alinear relation between force and elongation at least up to

b 250-600% elongation and must not possess significant hysteresis.

There are a group elastomers meeting the above requirements which havebeen found suitable for use in manufacturing expansible bodies accordingto the invention. Such elastomers are included within the group ofmaterials called segmented polyurethanes (PUR), several of which arecommercially available under trade names such as Pelethane (UpJohn),Biomer (Ethicon), Estane Goodrich. These materials can be dissolved insuitable solvents to form solutions, from which thin elastic bands orthin-walled tubes can be prepared for attachment to the supportingthread elements of helix configuration forming the framework of thebody.

When using prosthesis according to the invention as so-called grafts orvascular prostheses the wall of the prothesis, as previously mentioned,should be porous, thin and compatible with tissue and be composed so asto enable growth of natural tissue, inter alia neointima. Segmentedpolyurethanes (PUR) are also suited for use to form such walls since thesaid properties can be combined with the requirement of a wall having avery high elasticity. Such walls may be prepared in the form of a thintube consisting of fibres of segmented PUR formed by extrusion from asolution of PUR. The fibres are attached to each other at the crossingpoints and the wall can be made with the desired porosity by suitableadjustment of for example fibre thickness and density. The resultingtube can surround the body or can be attahced to the inside thereof.Alternatively, the thread elements of the body can be amalgamated withthe tube material, suitably when preparing the tube.

In order to impart the desired expansional force to a vascularprosthesis bands of PUr may be combined with suitable porous wallmaterial which can consist of monofilaments or multifilaments interwovenbetween the thread elements of the body or which can consist of a porouselastic wall prepared according to what has been described above.

In certain casses it may be suitable to make the body or its bands,sleeve or membrane from a biologically degradable material, for examplepolylactide and/or polyurethane.

Below there are given non-limiting examples of embodiments wherein theinventive principle has been applied.

EXAMPLE 1 Vascular graft

Expanded diameter 20 mms.

Angle α 160°.

Length 100 mms.

Suited for implntation in aorta within the diameter range 15 mms-18 mms.

Smallest diameter before implantation 8 mms.

Tota elongation about 300%.

Calculated axial force for fixation 0.1 kp provided by a microporouselastic PUR-wall having a thickness of 0.15 mms.

Pore size 15-50 μm.

Thread element material: polyester monofilament having a diameter of0.15 mms.

Number of elements n=72 (2×36).

EXAMPLE 2 Vascular prosthesis against stenosis

Expanded diameter 6 mms.

Angle α 100°.

Length 200 mms.

Implantation in veins within a diameter range 4-5 mms.

Total elongation 250%.

Axial force for expansion 0.08 kp provided by 4 elastic bands ofsegmented PUR, each having a width of 1.5 mms and a thickness of 0.3mms.

Thread element material: polypropylene monofilament having a diameter of0.09 mms and number of elements n=36 (2×18).

I claim:
 1. A method for forming and completely inserting a prosthesisin a body vessel, said prosthesis having a radially and axially flexibleelastic tubular body with a diameter that is variable under axialmovement of the ends of the body relative to each other and which iscomposed of a plurality of flexible and elastic thread elements, each ofwhich extends in helix configuration along the center line of the bodyas a common axis, the flexible and elastic elements defining a radiallyself-expanding body, said self-expanding body provided by a first numberof elements having a common direction of winding but being axiallydisplaced relative to each other and crossing a second number ofelements also axially displaced relative to each other but having anopposite direction of winding, said method comprising the steps of:(a)crossing the first and second elements such that an axially directedangle between the crossing elements is greater than 90°, the axiallydirected angle being defined by the crossing of the first and secondelements extending in the direction of the longitudinal axis of thecylinder; (b) holding the prosthesis in a contracted state; (c)inserting the prosthesis in its entirety into the body vessel at a firstlocation; (d) transferring the prosthesis in its entirety to a secondlocation, remote from said first location, in the vessel; and (e)allowing the prosthesis to expand within the vessel at said secondlocation so as to fixedly implant said prosthesis at said secondlocation.
 2. A method according to claim 1 including the step ofselecting a prosthesis of a diameter in unloaded state somewhat largerthan the internal diameter of the vessel at said other location so as toprovide for internal pressure of the prosthesis against the inner wallsof the vessel.
 3. A prosthesis prepared by a process by which theprothesis is formed and completely inserted into a body vessel at afixed location, said prosthesis having a radially and axially flexibleelastic tubular body with a diameter that is variable under axialmovement of the ends of the body relative to each other and which iscomposed of a plurality of flexible and elastic thread elements, each ofwhich extends in helix configuration along the center line of the bodyas a common axis, the flexible and elastic elements defining a radiallyself-expanding body, said self-expanding body provided by a first numberof elements having a common direction of winding but being axiallydisplaced relative to each other and crossing a second number ofelements also axially displaced relative to each other but having anopposite direction of winding, said process comprising the steps of :(a)crossing the first and second elements such that an axially directedangle between the crossing elements is greater than 90°, the axiallydirected angle being defined by the crossing of the first and secondelements extending in the direction of the longitudinal axis of thecylinder and completely form said prosthesis; (b) holding the prosthesisin a contracted state; (c) introducing the prosthesis in its entiretyinto the body vessel at a first location; (d) transferring theprosthesis in its entirety to a second location, remote from said firstlocation, in the vessel; and (e) allowing the prosthesis to expandwithin the vessel at said second location so as to fixedly implant saidprosthesis at said second location and complete said insertion process.4. The process of forming the prosthesis of claim 3 including selectinga prosthesis of a diameter in an unloaded state which is larger than aninternal diameter of the vessel at said second location so as to providefor internal pressure of the prosthesis against inner walls of thevessel.
 5. A method for using a prosthesis in a body vessel, saidprosthesis having a radially and axially flexible elastic tubular bodywith a diameter that is variable under axial movement of the ends of thebody relative to each other and which is composed of a plurality offlexible and elastic thread elements, each of which extends in helixconfiguration along the center line of the body as a common axis, theflexible and elastic elements defining a radially self-expanding body,said self-expanding body provided by a first number of elements having acommon direction of winding but being axially displaced relative to eachother and crossing a second number of elements also axially displacedrelative to each other but having an opposite direction of winding, saidmethod of use comprising the steps of:(a) crossing the first and secondelements such that an axially directed angle between the crossingelements is greater than 90°, the axially directed angle being definedby the crossing of the first and second elements extending in thedirection of the longitudinal axis of the cylinder; (b) holding theprosthesis in a contracted state; (c) inserting the prosthesis in itsentirety into the body vessel at a first location; (d) transferring theprosthesis in its entirety to a second location, remote from said firstlocation, in the vessel; and (e) allowing the prosthesis to expandwithin the vessel at said second location so as to fixedly implant saidprosthesis at said second location.
 6. The use of the prosthesis ofclaim 5, further comprising the step of selecting a prosthesis of adiameter in an unloaded state which is larger than an internal diameterof the vessel at said second location so as to provide for internalpressure of the prosthesis against inner walls of the vessel.